An arrangement having a laser diode for spectroscopic measurements of the concentration of a gaseous component is disclosed in the paper of M. Kroll et al entitled "Measurement of gaseous oxygen using diode laser spectroscopy" published in the Applied Physics Letters 51 (18), Nov. 2, 1987, pages 1465 to 1467. The method disclosed here relates to the gas spectroscopic measurement of oxygen concentrations on the basis of the radiation absorption in the wavelength range of approximately 760 nanometers. A laser diode of type ML-4405 having a monitor diode is used as the radiation source in the wavelength range of 759 to 764 nanometers. The laser diode is supplied by a control current comprising a direct-current component and an alternating-current component having a frequency of 5 KHz. The operating current I of the laser diode is adjusted with the direct-current component; whereas, the alternating-current component effects a periodic detuning in the region of the absorption lines. An alternating current having a sinusoidal curve trace is selected in order to provide a drive of the laser diode which is as free as possible of harmonics. The laser diode and the monitor diode are mounted as a block on a thermostat unit. By adjusting the temperature of the block, the laser diode is brought to one of the known absorption lines of oxygen. The operating point lies within the absorption line and is usually on the mean extreme value of the second derivative of the absorption line.
After passing through the gas to be investigated, the radiation emitted by the laser diode impinges upon a detector unit which is connected to an evaluation circuit. The evaluation circuit comprises essentially a lock-in amplifier having a signal input and a reference input. The measuring signal of the detector unit is applied to the signal input of this amplifier while a signal voltage having twice the frequency of the drive of the laser diode is applied to the reference input. The evaluation circuit further comprises a difference amplifier which likewise is connected to receive the measuring signal of the detector unit and to receive a signal from the monitor diode which is proportional to the radiation energy. The output voltage of the difference amplifier corresponds to the absorption line for the measured oxygen concentration. Since the absorption line is weakly pronounced especially for oxygen, the second derivative of the absorption line is used for the concentration measurement and corresponds to the output signal of the lock-in amplifier. The maximum amplitude of the output signal of the lock-in amplifier is approximately proportional to the partial pressure of the oxygen.
It is a disadvantage in the known arrangement that the laser diode can change the longitudinal mode when there is a change of the temperature T of the thermostat unit, a change of the operating current I and especially when there is a change because of deterioration processes associated with aging. This change in longitudinal mode causes a sudden frequency jump called mode jump. When manufacturing arrangements of this kind, mode jumps of the kind referred to above can be detected with an appropriate spectrometer and the operating point of the laser diode can be correspondingly adjusted. An adjustment of this kind can be carried out during the operating time only with a very substantial effort with respect to time and cost since the arrangement must then be connected to an appropriate spectrometer with which the spectrum of the laser diode can be recorded.
The paper of D. M. Bruce et al entitled "Detection of oxygen using short external cavity GaAs semiconductor diode lasers" published in Applied Optics, Volume 29, Number 9, Mar. 20, 1990, pages 1327 to 1332, discloses an arrangement wherein the mode jumps of the laser diode can be suppressed by means of an additional resonator which can be changed in length.
It is disadvantageous in this known arrangement that its realization is complex and requires a resonator as an additional component which disadvantageously affects the cost of the arrangement.